Bituminous coals on emergent surfaces in an Asbian, lower Carboniferous (Mississippian) limestone succession on the North Wales carbonate platform, UK, and implications for palaeoclimate

Abstract

Emergent surfaces in the Mississippian (Asbian to Brigantian) carbonate platform succession of North Wales record periods of plant colonization and peat formation that led ultimately to the local development of coals. Examination of bituminous coals on three emergent surfaces within Cefn Mawr Quarry reveals information on palaeoclimate that is not available from study of the limestones alone. Three coal seams in the Asbian Loggerheads Limestone Formation were identified and the lowest one studied in detail. Vitrinite reflectance data from alternating bands of vitrite and duroclarite microlithotypes, the distribution of pyrite within them, and the sharp contacts between them, suggest that there were abrupt changes in marine influence during the development of the peats that formed the coals. It is inferred that local palaeoclimate alternated between periods of high and low rainfall, the amount of rainfall influencing the extent to which seawater encroached into the peats, with higher rainfall suppressing the ingress of saline waters into groundwater. On the basis of modern peat growth rates, the timescale of the alternation indicated by each duroclarite-vitrite couplet is suggestive of an annual cycle, such as would arise in a monsoonal climate. The low proportion of ash in the three coals, the preservation of internal lamination, the low diversity of spore species in the lowest coal compared with the over- and underlying mudrock, and the presence of rhizoconcretions in palaeokarstic limestone beneath the lowest and highest coals, demonstrate that the peat swamps were isolated from the hinterland and autochthonous. This study demonstrates that a wider application of palynology and coal petrology is an important contribution to the study of marine carbonate successions of any age where terrestrial organic matter, formed during emergence, has been preserved.